There is a growing demand for conducting formation pressure measurements, especially in low-mobility environments (less than 0.1 mD cp−1). A tool used to conduct formation pressure measurements downhole is a formation tester such as the MDT™ (a trademark of Schlumberger) Modular Formation Dynamics Tester that determines the formation pore pressure and estimates the formation mobility (permeability/viscosity) and can collect samples of reservoir fluids. One challenge in the use of formation testers in low-mobility reservoirs is that because equilibration time is inversely proportional to the formation mobility, existing tools require a long time (up to several hours) for the pressure signal to equilibrate to the formation pressure. Moreover, equilibration is desirable for each pressure measurement, and measurements are made at several depths along a wellbore. In field operations, long waiting times with a stationary tool are undesirable, as they increase both the rig time and the risk of differential tool sticking. However, the information that formation testers can deliver is sufficiently valuable to operators that many are willing to wait, even hours, for the tool pressure to equilibrate to formation pressure if there is a guarantee that they will obtain good quality data.
Because of the long pressure equilibration times required for testing low mobility reservoirs, it is of commercial importance to implement robust real-time techniques to evaluate the quality of a test. In order to make an efficient use of the limited time available to evaluate the formation, it is desirable to assess as soon as possible whether it is worth waiting for the pressure signal to equilibrate, and if the field operations demand an early termination of the test, to at least extract the maximum amount of information from the data collected. Pressure measurement while drilling, where the control of the pretest is very limited, can also benefit from an assessment in real time as to the quality of the data being obtained.
The basic component of a formation tester for measuring the formation pore pressure is the tool flowline, which generally comprises a probe, a probe packer, a pretest piston, and a pressure sensor, all of which are connected by tubing. A formation tester pressure measurement starts when the tool is stationed in the wellbore at the desired depth and the probe is extended to make contact with the formation. In order to hydraulically isolate the probe and the formation from the wellbore, it is important that the packer makes a seal. After making a seal, in some tool designs, a piston that covers the probe orifice, known as the filter valve piston, is withdrawn. The filter valve piston is adapted to minimize the ingestion of solids in the tool flowline.
The pretest itself starts when a command is given to withdraw a pretest piston at a prescribed speed, qpiston, to increase the flowline volume by a prescribed amount, ΔV. This is the drawdown period. The increase in the flowline volume causes a decrease in the flowline pressure, Pfl. Once the pretest piston stops, Pfl increases until it equilibrates to the formation pore-pressure. This is known as the buildup period. The flowline pressure at the end of the drawdown and the rate of pressure change during buildup depend on the pretest parameters, qpiston and ΔV, on formation properties (mobility (k/μ), and compressibility), and on the tool design (size of the probe orifice, flowline dead volume and flowline compressibility (ceff)).